While men tend to maintain a woman as their closest confidant throughout their adult lives, women's focus shifts from their spouse to their adult daughter as they age, according to an analysis of nearly 2 billion cell phone calls and almost half a billion text messages.
The findings, released Thursday by the journal Scientific Reports, suggest that women's urge to ensure the survival of their genes may be connected with the nature of, and shift in, these intimate relationships.
Identifying the most important people in someone's life is a tricky task for researchers, said Ruth Mace, an evolutionary anthropologist at University College London who was not involved in the study.
"It's not politically correct to talk about some of these things," Mace said. "If I said, 'Were you equally close to your son or your daughter?' you wouldn't want to answer that question.
"But if you're looking at phone calls," she added, "you're getting a statistical picture that is quite unbiased."
That's why an international group of researchers obtained electronic communication records from 3.2 million customers of a mobile phone carrier in an unnamed European country. They looked for patterns among 1.95 billion calls and 489 million text messages over a seven-month period, noting the age and gender of the participants. The two contacts each person called and texted most often were deemed to be their first and second "best friends."
"No. 1 is very easy to distinguish," said physicist Vasyl Palchykov of the Aalto University School of Science in Finland.
Palchykov and his colleagues from Oxford University in England and Northeastern University in Boston found that in early adulthood, men and women tended to focus most of their attention on a member of the opposite sex -- presumably their romantic partner.
Women seemed to focus on their significant others at the age of 18, about four years earlier than men. The intensity of the relationship peaked earlier, too -- at age 27, as opposed to age 32 for men -- and lasted for about 14 years, twice as long as for men, the wireless records indicated.
"Females invest more heavily in opposite-sex relationships," Palchykov said.
But then, a twist: In their 40s, women's most important relationship began to shift away from the same-aged male to a female about 25 years or so younger -- presumably, her adult daughter. The strength of this relationship grew over the next 15 years or so -- possibly reflecting the gradual onset of grandchildren -- and peaked around age 60.
At the same time, women's second-best-friend slot became increasingly male -- probably indicating that the husband had been relegated to second-place status, the researchers said.
Men, on the other hand, appeared to be more stable in their mobile communications, the scientists found. They tended to stick with a female "best friend" -- their spouse, presumably -- for the duration. And when it came to second-best friends, they were remarkably gender-neutral and didn't appear to have a strong preference for either their sons or daughters.
Although the data were very 21st century, the conclusions are nothing new from an evolutionary standpoint, Mace said. "We also know from traditional populations that men don't get involved in child care so much, so they're not so involved in helping with the grandchild," she said. "I don't think we've changed that much."
Mace worked on a study of families in rural Gambia that found that maternal grandmothers -- mothers' mothers -- played a key role in the survival of their grandchildren. "Grandmothers were materially helping daughters raise grandchildren and actually helping keep them alive," she said. On the other hand, whether "your grandfather was dead or not didn't make much difference."
But this doesn't necessarily mean that men's communication lacks a reproductive strategy, she added.
"Men are capable of reproducing much later in life than females, so you might predict they'd still be more interested in talking to females that they were romantically involved with," she pointed out.
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